The invention relates to a powder coating system. More particularly, the invention relates to a powder coating system containing at least one adjustable throttle unit throttling the flow in at least one compressed air duct connected to a powder flow path along which the powder is pneumatically conveyed, and wherein, for each throttle unit, the system contains one electric motor connected to drive a rotating adjustment shaft of the throttle.
A powder coating system of this kind is already known from the German patent document 44 09 493A1 and in a similar way from the U.S. Pat. No. 3,625,404. Its throttle unit contains two throttles selectively adjusted manually or by an adjusting motor. Each of the throttles consists of a valve seat and a valve body mounted opposite each other. The two valve bodies are connected to each other. In this manner one throttle is being opened to the extent the other one is being closed when a shaft connected to them is axially adjusted, either manually or by the adjusting motor, by being rotated inside a thread.
The European patent document 0 297 309 B1 discloses a powder coating system wherein a flow throttle adjusted by its own control drive is mounted in a conveying air duct and in a supplementary air duct. Both ducts are connected on one hand to a source of compressed air and on the other hand to an injector implementing pneumatic powder conveyance. The conveying air generates a partial vacuum in the injector and in this manner aspirates powder out of a powder container. If more powder per unit time must be conveyed, a larger partial vacuum or suction is required and is produced by a commensurately adjusted larger flow of conveyance air. In order to assure that an approximately constant rate of air shall flow inside the powder duct pneumatically conveying the powder when the rate of conveyance air is raised or lowered, the additional air must be decreased when raising the conveyance air, and vice versa. Excessive air in the powder duct leads to blowing powder off the object being coated. Insufficient air entails powder pulses and powder deposits in the powder duct. An electronic control regulates the adjustment of the two throttles as a function of the quantity of powder per unit time being conveyed. The two throttles are not interconnected mechanically, but one linked only by the electronic control.
The adjustment shaft of the adjusted flow throttle is rotatable and implements axial adjustment displacements. Therefore, when using an electric motor to rotate the adjustment shaft, an axially variable connection is required between the adjustment shaft and a motor shaft in the event the motor shaft cannot be shifted adequately in the axial direction. Any desired throttle unit requires a corresponding rotation or a change in angle of rotation of the motor shaft. On account of the electric motors starts and stops, such throttle units will generate clickety-clack noises. Any electric motor is suitable as the adjustment drive of the flow throttle provided said motor shall offer accurate angular speeds and angular positioning as function of the drive applied by an electronic control unit.
The objective of the invention is to use an electric motor to implement in simple and economical manner accurate, low-noise and reproducible adjustment motions of the adjustment shaft.
Accordingly, the above problem is solved by the electric motor driving through a bellows connector the adjustment shaft of the throttle unit and in that a drive shaft of the stepping motor, the bellows and the adjustment shaft of the throttle unit are configured in axial manner.
Any type of electric motor will be appropriate which when electrically driven is able to carry out defined rotations, for instance DC motors, in particular however stepping motors and motor/gearing units wherein the gear reduces the angular motor speed are well suited.
The invention offers the following advantages, namely low noise and accurate and reproducible adjustment of the throttle unit. A stepping motor can be rotated in simple manner by electric pulses into an angular displacement corresponding to one step. Each step corresponds to a given throttle position. Because the number of steps required for any throttle adjustment can be predetermined, each throttle adjustment can be accurately reset any time. When using separate throttles for separate compressed air ducts, the air flow in each compressed air duct can be set individually and accurately. Preferably an electronic control unit is used for that purpose which can be preprogrammed with reference values for the rate of powder conveyed and/or the commensurate required air flows. The adjustable element of the throttle is mechanically connected to an adjustment shaft axially displaceable inside a thread in order to move the adjustable element back and forth. The adjustable element of the throttle, which ordinarily is not a valve seat but instead a valve cone, is correspondingly moved back and forth through the throttle""s thread. This axial displacement must be compensated relative to the axially stationary drive shaft of the stepping motor. This axial compensation is implemented in the invention by the simple design of a bellows connector. Moreover this bellows connector also effectively damps the noise generated by the stepping motor rotating in abrupt small steps. In the absence of the bellows connector, said steps of the said stepping motor would entail a more than trivial noise pollution. Said bellows automatically compensates any small, angular, axial and/or radial shift between the stepping-motor""s drive shaft and the adjustment shaft of the throttle as caused by manufacturing tolerances or in assembly. The system as a whole can be manufactured using simple, commercial elements and therefore it is also economical. The bellows of the bellows connector exhibits the property of being torsionally inelastic but damping and being comparatively compliant to axial loads. The bellows of the bellows connector may be made of any flexible material, preferably an elastically compressible material, also preferably of rubber.